This invention relates to a method and apparatus for use in surveying boreholes.
In the field of offshore oil drilling, considerations of maximising recovery and reducing costs have led to the extensive use of highly deviated drilling which will typically seek to follow thin, generally horizontal oil bearing strata. This has caused a requirement for increasingly sophisticated downhole instrumentation to be included in the drill string, both for borehole logging and for navigational purposes.
In addition to the present position of the drill bit, the driller has a need to know the present orientation of the far end of the drill string in relation to the earth. This is conventionally presented as Inclination (INC) and Azimuth (AZ) plus the rotational steering angles Highside (HS) and Magnetic Steering Angle (MS).
It is known to derive these measurement from an instrument package which has a set of three mutually perpendicular accelerometers and a set of three mutually perpendicular magnetic fluxgates. These provide respectively the gravity component set [Gx, Gy, Gz] and the magnetic flux component set [Bx, By, Bz] referenced to the instrument axis OZ aligned with the local borehole axis. The survey angles INC, AZ, HS and MS can then be derived by calculation from the two vector component sets.
However, the known technique requires the vector component to be measured with the drill string stationary. This is inconvenient, both in requiring cessation of drilling with attendant lost time, and in that the driller has no real time information while drilling.
An object of the present invention is accordingly to provide a method and apparatus which enable borehole survey data to be produced while the drill string is rotating.
The present invention, in one aspect, provides a method of determining the orientation of a borehole in which at least one parameter selected from gravity and magnetic flux is measured; the method comprising passing a sensor assembly along the borehole; rotating the sensor assembly; deriving from the sensor assembly, for the or each selected parameter, measurements of a first parameter component along the borehole axis and a second parameter component along an axis which is transverse to the borehole axis and rotates thereabout with rotation of the sensor assembly; and calculating, for the or each selected parameter, from the first and second parameter components the value of one or more desired orientation angles.
From another aspect, the present invention provides apparatus for determining the orientation of a borehole, comprising a sensor assembly adapted to be incorporated in a rotating drill string; the sensor assembly being capable of measuring at least one parameter selected from gravity and magnetic flux; the sensor assembly comprising, for the or each selected parameter, a first sensor having a detection axis arranged along the axis of the drill string for measuring an axial component of the parameter to provide an axial component signal, and a second sensor having a detection axis arranged transversely to the drill string axis for measuring a transverse parameter component in a direction which, in use, rotates with the drill string to provide a transverse component signal; and means for calculating from said component signals the value of one or more desired orientation angles.
Preferably, both parameters (gravity and magnetic flux) are measured, and the orientation angles produced are azimuth and inclination. The local magnetic dip angle may also be calculated. The transverse component is preferably radial, but could be at some angle other than 90xc2x0 to the borehole axis.
The calculation preferably comprises deriving for each revolution of the sensor assembly a train of a given number of equally spaced pulses, and using said pulse train to control integration of one or both of the gravity and magnetic flux signals over successive portions of each revolution, said portions preferably being quarter revolutions.
The integration may suitably be used to produce a set of simultaneous equations which are solved to derive the orientation angles.